The general goal of the proposed studies is to develop a more comprehensive model of the regulation of seasonal breeding in rodents. More specifically, the goals of this proposal are two-fold: (1) to identity more precisely the external cues that act alone or in concert with photoperiod to regulate seasonal breeding male deer mice and prairie voles, and (2) to ascertain the relative costs and benefits of seasonal reproductive quiescence. A number of factorial experiments are proposed to study the interactive effects of four environmental factors (photoperiod, temperature, food quality, and food quantity) on reproductive function. Previous studies on the control of rodent seasonal breeding have primarily focused upon the role of photoperiod (day length) influences upon breeding in animals maintained in mild laboratory ambient temperatures with ad libitum access to food and water. However, a substantial minority of deer mice and prairie voles fail to respond to inhibitory photoperiods; these animals display seasonal breeding patterns in the field. Data from these and other agricultural pest species (e.g., Microtus ochrogaster, M. pinetorum) support the contention that day length cues may be insufficient or of minimal significance in the regulation of seasonal reproduction in small, opportunistic breeders. Some individual deer mice inhibit reproduction when ambient temperatures are low, whereas other mice are affected only if food intake is reduced. Winter breeding rodents would appear to have an advantage over reproductively quiescent animals in terms of possible breeding successes. There must be costs to maintaining the reproductive system during the winter. Several experiments are proposed to evaluate the costs and benefits of winter reproduction in small rodents. The effects of extrinsic and social factors on metabolic rates and thermoregulatory behavior will be assessed. The proposed studies will provide data to understand more completely the hierarchy of regulatory extrinsic cues. These studies will provide new conceptual perspectives of seasonally breeding rodents. The use of naturally selected populations of rodents, rather than highly inbred species and the manipulation of several environmental factors, instead of the traditional one or two, should allow new, ecologically relevant statements about the control of seasonal reproduction.